McCaskill attended Edison High School in Huntington Beach his Freshman year before being accepted at Trinity-Pawling School in Pawling, New York. He move to the boarding school mainly to pursue his hockey career, but continued to play baseball while there.[2] During his senior year, McCaskill had an 8–0 record with an 0.97 ERA and 97 strikeouts, scored 26 goals and 22 assists in 17 hockey games, and was the varsity soccer team's leading goal-scorer.[2] He turned down a baseball scholarship to Arizona State University so that he could pursue both hockey and baseball at the University of Vermont.[2]

All-ECAC in baseball and hockey as a collegiate student-athlete, McCaskill was drafted in the fourth round of the 1982 amateur draft by the California Angels (88th overall), from the University of Vermont, and would be the first baseball player from UVM to reach the major leagues since Jack Lamabe in 1962. He would debut in 1985, but his break-out season was 1986, with the American League West champion California Angels. He compiled a win-loss record of 17–10, with a 3.36 earned run average, and 202 strikeouts. He also totaled ten complete games on the season, and would go on to have six seasons of ten or more wins, throw two one-hitters, rank in the American League top ten in shutouts and earned run average three times each. On September 14, 1990 Kirk was the pitcher of record when Ken Griffey Sr., and Ken Griffey, Jr became the first father son duo to hit back to back home runs. At the end of his career, he played for the Fiorentina Baseball team, in Florence, Italy.

In 2003, McCaskill was inducted into the Canadian Baseball Hall of Fame, along with Joe Carter, Richard Bélec, and the Asahi. In response to the news of induction, McCaskill was quoted as saying, "I am stunned by this wonderful news. I can't wait to tell my family and my parents. I am very proud of my Canadian heritage, and this is going to be an honor of a lifetime."

McCaskill played center and right wing for the University of Vermont from 1979 to 1983. In 1982, McCaskill was a finalist for the Hobey Baker Award and named to that season's NCAA East All-America First Team and the ECAC All-Star First Team. He was the team captain during the 1982–83 season, and won the Cunningham Award as the Most Valuable Player on the Catamounts.

McCaskill was drafted in the fourth round (64th overall) by the Winnipeg Jets in the 1981 NHL Entry Draft. He played only one season of professional hockey for the Sherbrooke Jets, a Jets farm team. McCaskill dressed for one game with the Winnipeg Jets of the NHL but did not play in the game. During the 1983–84 season, he scored 10 goals and added 12 assists for 22 points. He retired from professional hockey after the one season to focus on his professional baseball career.